One known prior art kiln, which is disclosed in DE 30 38 927 C2, operates cyclically, with burning being carried out only in the burning shaft while the other shaft operates as regenerative shaft. During a cycle, which takes, for example, 10-12 minutes, the carbonate rock to be burned is discharged continuously via discharge devices on both shafts. The column of material descends uniformly in the shafts. The operation of the kiln is then changed so that the shaft which has previously operated as burning shaft becomes the regenerative shaft and the shaft which has previously operated as regenerative shaft in turn becomes the burning shaft. The charging of the kiln with carbonate rock is carried out during the change of operation or during the burning process on the regenerative shaft. In the lower region of the two shafts, cooling zones for cooling the carbonate rock calcined in the upper region are provided, with part of the cooling air used for this purpose being fed in in the region of the discharge devices and part being fed in via a central displacement body arranged in each cooling zone.
The quality of the end product depends critically on the calcined carbonate rock descending very uniformly downward over the width of the shaft and the cooling zone being configured so that very uniform cooling is ensured. The above-described cocurrent regenerative lime kiln is known from the article by Hannes Piringer, “Schüttgutbewegung, Stromungsverhalten und Temperaturverteilung in der Kühlzone eines 800 und 1000 tato Maerz-Regenerativ-Kalkschachtofens”, Veitsch-Radex Rundschau 1/1999, pages 1 to 15. In addition, studies on an optimal configuration of the cooling zone and of the discharge devices are presented in detail.
It is known from industrial practice that in the case of cocurrent regenerative lime kilns, the angle of inclination in the cooling zone has to be very steep since, particularly in the case of small pieces of rock (e.g. from 10 to 50 mm), the material being burned no longer descends at the outer margin of the cooling zone and larger accumulations are formed as a result. A change has therefore been made in the case of the Maerz fine rock kilns to cylindrical or slightly conical cooling zones having an angle of inclination of more than 82°. Although this has been able to alleviate the problems of material buildup, other disadvantages occur as a result of this construction. Due to the construction, the exit diameter at the lower end of the cooling zone is significantly larger, so that it is difficult to distribute the cooling air uniformly in the cooling zone in order to cool the burned material uniformly. However, nonuniform cooling of the burned material results, in particular, in the burned material not being cooled sufficiently in the middle of the shaft, as a result of which damage can occur at the discharge devices. On the other hand, the burned material is cooled too greatly in the outer region, especially in the upper region of the cooling zone, as a result of which an unwanted reaction of the combustion exhaust gases with the burned material can occur and the burned material once again becomes enriched in CO2 from the combustion exhaust gases and the product quality is thereby impaired. To avoid local overheating, the specific amount of cooling air for cooling the burned material has therefore been increased, but this in turn also results in an increased exhaust gas temperature, which adversely affects the heat consumption. In addition, it has been proposed that the central displacement body be enlarged, which improves the cooling air distribution, especially in the lower part. In the case of large displacement bodies, the tip reaches virtually to the lower end of the burning zone. However, they are very complicated and expensive since they have to be made with a double wall and have to be continually cooled with air, even when the kiln is not in operation.
A change has therefore been made back to smaller displacement bodies which extend at most to half the height of the cooling zone. In order to achieve a very good cooling action, the major part of the cooling air, usually at least 85%, has been introduced via the discharge devices and the remainder of not more than 15% has been introduced via the central displacement bodies.